1. Field of the Invention
This invention relates generally to the method of latching a top plate commonly used in a testing apparatus for printed circuit boards.
2. Background of the Invention
It is not uncommon for automatic test fixtures to use a vacuum box system to test printed circuit boards. Generally, a printed circuit board is placed into a vacuum box fixture, where the test probes have been pre-set for the specific circuit board to be tested, and a top plate is placed over the board. A vacuum is applied and the circuit board comes into contact with the test probes, allowing the board's circuit components to be tested. Each test box must be customized for the specific circuit boards since each designed circuit board has the components and “wiring” in specific locations. Each vendor of a circuit board varies its design causing the requirement of differing setups of automatic test fixtures.
In the past, Top (Moving or Diaphragm) Plates were held in place by means of a removable Dress Frame. This was a five-sided piece of aluminum with the center removed except for about ½ inch on all four sides. The resulting picture frame like item contained two small holes in a front vertical wall and two small holes in a rear vertical wall. The resulting picture frame like item would slip over the Top and Bottom (Probe) plate stack and be held in position via four protruding pins, where the protruding pins would be aligned with the four holes in the front and rear vertical walls. The industry used this method for a long time but eventually moved away from this approach largely because of a new approach to sealing the void between the Top and Bottom plates. It should also be noted, that a common complaint about the removable dress frame was that once you removed it to allow removal of the Top Plate to access the Probe Plate, you would need to find a place to temporarily store this bulky dress frame, where people wouldn't trip over it or damage it.
The next generation of a Top Plate “hold down” method, was to employ two or four locking pins. These locking pins would be located either in two or four corners of the Top Plate. There are at least two known drawbacks to this approach. First, all of the current methods result in protrusions or bumps in two or four corners of the Top Plate, which in some cases can cause interference with other hardware that may need to be attached to the Top Plate. Second the current methods could result in partial release or engagements, which can then cause the top plate to hang up upon removal or installation.